1. Field of the Invention
The present invention relates to a solid state imaging device represented by a photoelectric conversion device such as a C-MOS, CCD, etc., to a method of manufacturing the solid state imaging device, and to a substrate for a solid state imaging device. In particular, the present invention relates to a solid state imaging device provided with a micro lens which is substantially free from surface roughness and small in inter-lens gap, to a method of manufacturing such a solid state imaging device, and to a substrate for the solid state imaging device.
2. Description of the Related Art
A region (aperture) of photoelectric conversion device such as a C-MOS, CCD, etc., which is designed to contribute to the photoelectric conversion is limited to about 20 to 40% based on the entire area of the photoelectric conversion device, though it depends on the size of elements as well as on the number of picture elements. Since limited size of aperture directly leads to the deterioration of photo-sensitivity thereof, there has been generally practiced, in order to complement the limited size of aperture, to provide the photoelectric conversion device with a micro lens for converging light.
Recently however, there has been an increasing demand for a very fine CCD imaging device where the number of picture elements exceeds over three million. In this very fine CCD imaging device, the decrease of aperture ratio of the micro lens (i.e., decrease of photosensitivity) mounted thereon as well as the deterioration in quality of image due to the increase of noise such as flare and smear are now becoming great issues to be dealt with. As far as the imaging device such as a C-MOS, CCD, etc., is concerned, the number of picture elements is now approaching to almost a sufficient degree so that attentions among the competitive device makers are now being shifted gradually from the number of picture elements to the quality of image. In order to enable the imaging device to mount on a portable telephone or small PDA equipments, the size of picture element of the imaging device is now being gradually miniaturized as small as around 2 μm.
As for the technique of forming the micro lens, Japanese Laid-open Patent Publication (Kokai) No. 60-53073 (1985), for example, describes relatively in detail about the technique of employing dry etching. In this Japanese Laid-open Patent Publication, there are also disclosed about the technique of forming the lens into a semi-spherical configuration by taking advantage of thermal fluidity of resin by using heating flow of resin as well as about the technique of forming an organic film such as a film of PGMA or an inorganic film such as a film of OCD (SiO2 type) on the surface of the lens.
Japanese Laid-open Patent Publication (Kokai) No. 6-37058 (1994), for example, describes in detail about the technique of forming a micro lens through transfer and working by dry etching and about the technique of using CF4 gas and O2 gas as an etching gas in the dry etching. Hereinafter, for the purpose of convenience, a micro lens to be prepared by dry etching technique will be referred to as transfer lens and this working method of lens will be referred to as a transfer method.
Due to the facts that it is possible, through the employment of the transfer lens, to make thinner a stack structure comprising a color filter and a planarization layer to be formed on a photoelectric conversion device and also to increase the aperture ratio of micro lens higher than that of heating flow lens, it can be said that the characteristics of the transfer lens resides in the capability thereof to enhance the performance of imaging device.
Further, there is described, in Japanese Laid-open Patent Publication (Kokai) No. 6-112459 (1994) and Japanese Laid-open Patent Publication (Kokai) No. 2003-229550 (2003) for example, techniques of forming transfer lens wherein a heating flow lens is formed on a layer of photosensitive lens material by using the same photosensitive lens material and the resultant heating flow lens is employed as a master to transfer the pattern of the master by dry etching, thereby forming the transfer lenses. Further, the technique of minimizing the gap between picture elements by using a photosensitive lens material is described in Japanese Laid-open Patent Publication (Kokai) No. 2000-269474 (2000) for example.
The method of forming a micro lens through the transfer of the configuration of lens master by dry etching is generally performed by a method wherein a lens master is created at first on an underlying transparent resin layer by using a photosensitive resinous material exhibiting heating flow property of resin and by sequentially subjecting the photosensitive resinous material to exposure, development and heating flow of resin, and then the pattern of lens master is transferred onto the underlying transparent resin layer by dry etching to form the transfer lenses.
In the creation of the micro lens according to this transfer method, the height of the transfer lens to be created depends on the etching rate of the underlying transparent resin layer as well as on the etching rate of the lens master.
Theoretically, it may be possible to control the configuration of lens through the combination of the transparent resin layer for forming the transfer lens with the etching rate of the lens master.
For example, if it is desired to make the configuration of lens lower than the lens master, it is conceivable to employ either a method of selecting a lens master which is larger in etching rate or a method of selecting a transparent resin which is smaller in etching rate. However, since the resin to be employed as a lens master is required to be excellent in heating flow property of resin as well as in photosensitivity so as to enable it form a pattern through exposure and development, the range of option is relatively narrow so that if the resin is selected based solely on the etching rate thereof, other characteristics thereof may be sacrificed. On the other hand, if a transparent resin which is smaller in etching rate such as styrene resin or phenol resin is selected, since these materials are low in heat resistance, i.e., at most 230° C. or so in heat resistance, and furthermore, since these materials are low in etching rate, the employment of these materials would lead to deterioration of productivity efficiency.
Whereas, a photosensitive resin excellent in heating flow property of resin is generally smaller in etching rate than the materials for forming the transfer lens (underlying transparent layer) which is desirable for use in terms of transparency and heat resistance. Therefore, when the lens master created from a photosensitive resin is transferred to a transparent resin layer, the resultant transfer lens thus obtained would become larger in height than the lens master and also become larger in gap (i.e. greater in inter-lens gap).
In this case, since the lens is rounded by taking advantage of the surface tension to be created by the heating flow of photosensitive resin, it is difficult to control the height of the lens master itself, so that it is impossible to make the lens master extremely high or low.
As for specific examples of the photoelectric conversion device to be essentially employed in order to enable a solid imaging device to function, CCD and C-MOS are known for example. While the CCD is advantageous in that it can be made into a fine structure, the C-MOS is advantageous in that the structure thereof is relatively simple and small in power consumption, so that they are properly employed depending on specific purposes. As for the distance from the surface of a semiconductor substrate to the photoelectric conversion device, it is relatively short in the case of the CCD but relatively long in the case of the C-MOS. Therefore, it is required, in accordance with their specific characteristics, to specifically select the focal distance of micro lens, i.e., the height of micro lens. However, as described above, in the case of transfer system, the height of the transfer lens to be obtained is determined based on the etching rate of the underlying transparent resin layer and on the etching rate of the lens master, thereby raising the problem that the configuration of lens cannot be controlled.
Further, in order to enhance the sensitivity and image quality, the aperture ratio of solid state imaging device is generally required to be as close to 100% as possible. Therefore, in order to achieve this aperture ratio, the distance between neighboring micro lens (hereinafter, referred to as an inter-lens gap) should preferably be as small as possible, or ideally the neighboring micro lens should be contacted with each other. However, there is a problem that, in the case of the micro lens to be formed by means of heating flow of resin, it is impossible to make the inter-lens gap extremely small because of the requirement to retain the configuration of lens.
Further, the solid state imaging device is required to exhibit a heat resistance of as high as 200° C. or more in the manufacturing process thereof. Recently in particular, there is an increasing demand that a semiconductor device comprising an imaging device is capable of being mounted using a lead-free solder and hence the semiconductor device is now required to exhibit a heat resistance ranging from 240 to 260° C.
Under the circumstances mentioned above, it is conceivable to employ acrylic resin which is a thermosetting resin as a resin excellent in heat resistance. The acrylic resin however is accompanied with the problem that the etching rate thereof is generally higher than that of the heating flow type photosensitive resin to be employed as a material for forming the lens master and hence the height of transfer lens to be created may become too high. Further, a resin which is excellent in heat resistance such as thermally cured acrylic resin is generally somewhat insufficient in dry etching resistance and the surface thereof is liable to become roughened, so that, if such a resin is employed, the surface of transfer lens would be greatly roughened, thereby causing the scattering of light and hence resulting in the deterioration of light transmittance and also in the deterioration in quality of image.
Furthermore, in conformity with the trend to miniaturize the solid state imaging device, it is also required to reduce the reflectance factor of incident light at the surface of micro lens, thereby reducing the re-incidence of the reflected light from the cover glass of the surface of solid state imaging device to enhance the quality of image (improvement of signal-to-noise ratio). As for the materials for forming such a micro lens, it is possible to preferably employ a resin of low refractive index such as fluorine-based acrylic resin, etc. However, a resin which is low in refractive index is more likely to generate a roughened surface due to the dry etching thereof. Therefore, when a transparent resin layer is formed by using the resin of low refractive index and a pattern of the lens master is transferred to the transparent resin layer, the surface of transfer lens to be obtained would be roughened much more, thus raising a problem.